The present invention pertains to a detection method and apparatus and, more particularly, to a method and apparatus for detecting the presence of "flat" wheels, that is, wheels having flat segments, on railroad cars.
A so-called wheel flat results if a wheel of a railroad car or vehicle is so braked or locked that instead of rolling it slides along a rail. When this happens the high friction which develops between the wheel and the rail produces flat segments or portions in a given wheel. It will further be understood that the majority of these wheel flats appear during the winter because it is at this time that the brake shoes have a tendency to freeze against the wheels which results in the aforesaid sliding and the development of wheel flats. Other kinds of brake faults can also result in wheel flats, even during mild weather.
Accordingly, it will be appreciated that if wheel flats are left unattended or not repaired they can cause serious and extensive damage to rails as well as produce high stress regions on the wheel so that it becomes important to detect such flat wheels on railroad cars in order that the cars may be taken out of service so soon as practicable for repair.
It is, therefore, a fundamental object of the present invention to accomplish such detection efficiently and economically.
Certain kinds of apparatus have been directed to the detection of wheel flats and an example of an automatic means or apparatus for detecting their presence may be understood by reference to U.S. Pat. No. 3,844,513 in which there is disclosed a system and method for detecting the presence of wheel flats, such system relying on the sensing of changes in voltage resulting from a break in an established circuit caused by a wheel flat. Also, in the aforementioned patent, reference is made, in a general way, to a known acoustical method in which sound from a passing train is recorded and the particular sound cause by the impact between a wheel flat and the supporting rail is distinguished by detecting frequencies in that particular sound. Such an acoustical method is indicated in that patent as being an impractical solution, although the reason therefor is not given.
The present invention represents an improvement in an acoustical method of detecting flat wheels on railroad cars, it being a primary object of the invention to efficiently provide output signals indicative of the presence of flat wheels and to do so automatically and in conditions of poor signal-to-noise ratios. The inability previously to operate under such conditions is believed to be the major reason for the impracticability of prior art acoustical techniques for detecting wheel flats.
The present invention is based on the principle of effectively discriminating against the noise present in an acoustical signal that is picked up from the environment by adaptively filtering the acoustical signal so as to enhance significantly the signal-to-noise ratio, such adaptive filtering being made responsive to the particular wheel diameter and speed of the advancing railroad car or cars.
Accordingly, the present invention in its broadest terms resides in the provision of a system or method for acoustically detecting the presence of wheel flats, such system comprising an electro-acoustic transducer for picking up the sounds generated by a passing train; a demodulator or detector; a means for limiting the band width, for example, by a suitable external filter, so as to limit or restrict the band under consideration to that in which the normal periodic clanging sound of a wheel flat occurs; a sampling analog-to-digital converter operating at a frequency of approximately 200 Hz, in connection with a means for removing the DC or average value of the picked up signal; and a digital first order adaptive filter, connected to the analog-to-digital converter, and being provided with a filter adjusting input signal determined by the wheel diameter and the train speed, such that as a result the adaptive, or programmable, filter cuts off at a frequency which is approximately twice the frequency of interest, thereby allowing for some wheel size variation and train speed changes.
A primary feature, in addition to the above recited combination of elements, lies in the fact that an autocorrelator is utilized for performing autocorrelation with respect to predetermined signal samples received from the adaptive filter, such operation involving autocorrelating with respect to each of the predetermined signal samples and of the ten samples preceding each of the predetermined samples. More specifically, a current sampled and filtered signal value is obtained for every one-fifth of a wheel period; each sample value is stored in memory and is correlated with the aforenoted previously stored samples in memory.
Associated with the autocorrelator is a sampling gate which receives an input from a train wheel revolution period generator which provides a gating signal representative of one-fifth the period of the flat spot occurring on a train wheel. In other words, for any given train speed a gating signal is provided at every one-fifth of a wheel revolution. In accordance with the particular period of an occurring flat spot, the signal transmitted from the adaptive filter is gated through to the autocorrelator. The precise way in which this is effected will be described hereinafter.
A further feature resides in the provision of an interpolation means connected to the digital first order adaptive filter such that the 200 Hz sampler has the effect of a 2000 Hz sampler. Thus, the interpolation means permits implementation of the invention is a low cost, slow speed, signal processing system, which could take the form of a microprocessor currently on the market, some of which cost as little as twenty dollars in shipped form.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.